Migraine is a highly disabling neurovascular disorder characterized by attacks of moderate to severe headache that are often associated with nausea, vomiting, photophobia, and phonophobia. The attacks can last from 4 to 72 h, and the average attack frequency is 1 or 2 per month. About 20-30% of migraine patients experience transient focal neurologic symptoms known as aura, which are usually visual and can precede or accompany the headache. Migraine afflicts about 11% of adults worldwide and results in a significant socioeconomic burden, in terms of both quality of life and lost productivity.
Whilst the pathomechanism of migraine is still unclear, one of the leading hypotheses is based on activation of the trigeminovascular system (TS). Several neuropeptides participate in this activation, calcitonin gene-related peptide (CGRP) playing a crucial role among them. CGRP exerts various biological effects through the peripheral and central nervous system (CNS). The functional CGRP-receptor (CGRP-R) complex has been well characterized, and novel therapeutic approaches target CGRP itself and its receptors. This invention relates to the development of CGRP receptor antagonists (CGRP-RA).
CGRP, a 37-amino acid neuropeptide derived from the gene encoding calcitonin, is formed from the alternative splicing of the calcitonin/CGRP gene located on chromosome 11. In humans, CGRP has two isoforms: α- and β-CGRP. The β-isoform differs from the α-isoform in the amino acids located at positions 3, 22 and 25. The chemical structure of CGRP involves a disulphide bridge between residues 2 and 7 and an amidated C-terminus. The cyclic cysteine2-cysteine7 motif has a basic role in receptor activation. In the human trigeminal ganglia (TRIG), CGRP-immunoreactive neurons account for up to 50% of all neurons. It has been demonstrated through an in situ hybridization technique that 40% of all nerve cell bodies contain CGRP mRNA and CGRP. Double immunostaining has shown that in the human TRIG CGRP is co-localized with nitric oxide synthase, substance P (SP), pituitary adenylate cyclase activating peptide (PACAP) and nociceptin, which may play a role in the pathomechanism of migraine.
The functional CGRP-R consists of three proteins: i) Calcitonin Receptor Like Receptor (known as CRLR, CALCRL or CLR) is a seven-transmembrane spanning protein, which forms the ligand binding site with; ii) RAMP1, determining the specificity of the receptor; and iii) the CGRP-R component protein (RCP) couples the receptor to intracellular signal transduction pathways and to adenylyl cyclase.
It is thought that the C-terminal region of CGRP initially binds to the large N-terminal extracellular domain (ECD) of the receptor, likely making interactions with both CLR and RAMP1. This initial binding event greatly increases the local concentration of the N-terminal region of CGRP in the vicinity of the juxtamembrane portion of CLR, allowing their relatively weak interaction to occur and resulting in receptor activation. Since mutagenesis experiments indicated that most small molecule antagonists interacted with the ECD of CLR/RAMP1, it was hypothesized that they bind to this region of the receptor and prevent the initial binding of CGRP to the receptor. A notable exception to this model of peptide binding and small molecule receptor antagonism is the hydroxypyridine class of antagonists, which apparently interact with transmembrane domain 7 (TM7) in CLR and not with the extracellular domain (Bell I M, J. Med. Chem., 2014, 57(19), 7838-58).
The first clinically tested CGRP-RA, olcegepant, was based on a dipeptide backbone, had high molecular weight, and was not orally bioavailable. Nonetheless, when dosed intravenously, olcegepant proved to be an effective antimigraine agent, and this proof-of-concept study greatly increased interest in the field. Following the success of olcegepant, a number of orally acting CGRP-RAs were advanced to clinical trials. Telcagepant and compounds BI 44370, MK-3207, and BMS-927711 have all been used for acute treatment of migraine as oral agents. Taken together, the results from these clinical studies demonstrate that CGRP-RAs can exhibit similar antimigraine efficacy to the gold standard triptan drugs but with a significantly lower incidence of adverse events than is typically observed with a triptan. It is worth noting that the available data indicate that these CGRP blockers do not cause vasoconstriction and suggest that they may have a superior cardiovascular safety profile to the triptans. One potential concern that has been reported with some CGRP-RAs is the observation of elevated levels of liver transaminases in some patients, and this reportedly led to the discontinuation of MK-3207. Although elevated liver enzymes were also found in a small number of subjects after dosing with telcagepant for an extended period, it is not clear if these findings are in some way mechanism-based or specific to these two compounds. In clinical trials for acute migraine therapy, the CGRP-RAs displayed favorable effects, but their frequent administration was associated with liver toxicity (the elevation of liver transaminases), which limited their clinical use. Hence, there is a need to develop new CGRP-RAs which do not induce liver injury.